Electrical submersible pumps (ESP) are often employed to pump well fluid from wells. A typical ESP includes a rotary pump driven by an electrical motor. Normally, the ESP is suspended in the well on a string of production tubing. A seal section, usually located between the motor and the pump, has a movable element to reduce a pressure differential between the well fluid exterior of the motor and motor lubricant contained in the motor. The pump may be a centrifugal pump having a plurality of stages, each stage having an impeller and a diffuser.
Some wells produce gas along with liquid, and centrifugal pumps operate best when pumping primarily liquid. Gas separators of various types may be employed to separate the gas from the liquid prior to reaching the pump. However, some gas may still reach the pump, particularly when the well fluid contains slugs or large bubbles of gas.
Shrouds may be employed in various ways to cause gas separation before reaching the pump intake. In one design, the shroud surrounds the pump and has an inlet at an upper end. Well fluid flows upward around the shroud, then downward into the inlet and to the pump intake. As the well fluid turns to flow downward, gas in the well fluid tends to continue flowing upward while the heavier liquid portions flow downward into the shroud inlet. Having a large enough annulus between the shroud and casing can be a problem with some wells.
U.S. Pat. No. 6,932,160 discloses system using a riser offset from a longitudinal axis of the ESP. The riser has an inlet extending through a barrier in the well below the pump intake. The riser has an outlet above the pump intake. As well fluid discharges from the bypass tube outlet, the gas portions tend to continue flowing upward while the liquid portions flow downward to the pump intake. The bypass tube may have helical vanes within to enhance separation of the gas and liquid portions.